1. Field of the Invention
This invention relates to an electro-luminescence display (ELD), and more particularly to an electro-luminescence display device with a high aperture ratio and a driving method thereof.
2. Discussion of the Related Art
Recently, various flat panel display devices have been developed with reduced weight and size that are capable of eliminating the disadvantages associated with a cathode ray tube (CRT). Such flat panel display devices include liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP) and electro-luminescence (EL) panels.
The EL display in such display devices is a self-emission device in which a phosphorous material is excited using recombination of electrons and holes. The EL display device is generally classified into inorganic EL devices and organic EL devices, depending upon a source material for the light-emitting layer. The EL display has the same advantage as the CRT in that it has a faster response speed than passive-type light-emitting devices requiring a separate light source like the LCD.
FIG. 1 is a cross-sectional view showing a related art organic EL structure for explaining a light-emitting principle of the EL display device.
Referring to FIG. 1, the organic EL device includes an electron injection layer 4, an electron carrier layer 6, a light-emitting layer 8, a hole carrier layer 10 and a hole injection layer 12 that are sequentially disposed between a cathode 2 and an anode 14.
If a voltage is applied between a transparent electrode, that is, the anode 14 and a metal electrode, that is, the cathode 2, then electrons produced from the cathode 2 are moved, via the electron injection layer 4 and the electron carrier layer 6, into the light-emitting layer 8, while holes produced from the anode 14 are moved, via the hole injection layer 12 and the hole carrier layer 10, into the light-emitting layer 10. Thus, the electrons and the holes fed from the electron carrier layer 6 and the hole carrier layer 10, respectively, collide at the light-emitting layer 8 to be recombined to generate a light. This light is emitted, via the transparent electrode (i.e., the anode 14), into the exterior to thereby display a picture.
FIG. 2 shows a related art active matrix type EL display device.
Referring to FIG. 2, the related art active matrix type EL display device includes an EL display panel 16 having pixel (hereinafter referred briefly to as “PE”) cells 22 arranged at each intersection between gate electrode lines GL and data electrode lines DL, first and second gate drivers 18 and 19 for driving the gate electrode lines GL, and a data driver 20 for driving the data electrode lines DL. The first gate driver 18 sequentially applies a first gate signal to odd-numbered gate electrode lines GL1, GL3, . . . GLn−1. The second gate driver 19 sequentially applies a second gate signal to even-numbered gate electrode lines GL2, GL4, . . . GLn. Herein, the first and second gate signals are set to have the same width (e.g., 1H), and are applied in such a manner to overlap with each other during a predetermined period.
The data driver 20 applies video signals corresponding to a data, via the data electrode lines DL, to the PE cells 22. In this case, the data driver 20 applies the video signals for each one horizontal line to the data electrode lines DL every one horizontal period when the first and second gate signals are supplied.
The PE cells 22 generate a light corresponding to the video signals (i.e., current signals) applied to the data electrode lines DL to thereby display a picture corresponding to the video signals. To this end, as shown in FIG. 3, each PE cell 22 includes a light-emitting cell driving circuit 30 for driving a light-emitting cell OLED in response to a driving signal supplied from each of the data electrode lines DL and the gate electrode lines GL, and a light-emitting cell OLED connected between the light-emitting cell driving circuit 30 and the ground voltage source GND.
The light-emitting cell driving circuit 30 includes a first driving thin film transistor (TFT) T1 connected between the supply voltage line VDD and the light-emitting cell OELD, a first switching TFT T3 connected between the odd-numbered gate electrode line GLo and the data electrode line DL, a second switching TFT T4 connected between the first switching TFT T3 and the even-numbered gate electrode line GL, a second driving TFT T2 connected between a node positioned between the first and second switching TFTs T3 and T4 and the supply voltage line VDD to form a current mirror circuit with respect to the driving TFT T1, and a storage capacitor Cst connected between a node positioned between the first and second driving TFTs T1 and T2 and the supply voltage line VDD. Herein, the TFT is a p-type electron metal-oxide semiconductor field effect transistor (MOSFET).
A gate terminal of the driving TFT T1 is connected to the gate terminal of the second driving TFT T2; a source terminal thereof is connected to the supply voltage line VDD; and a drain terminal thereof is connected to the light-emitting cell OLED. A source terminal of the second driving TFT T2 is connected to the supply voltage line VDD, and a drain terminal thereof is connected to a drain terminal of the first switching TFT T3 and a source terminal of the second switching TFT T4.
A source terminal of the first switching TFT T3 is connected to the data electrode line DL, and a gate terminal thereof is connected to the odd-numbered gate electrode line GLo. A drain terminal of the second switching TFT T4 is connected to the gate terminals of the first and second driving TFTs T1 and T2 and the storage capacitor Cst. A gate terminal of the second switching TFT T4 is connected to the even-numbered gate electrode line GLe.
Herein, the first and second driving TFTs T1 and T2 are connected to each other in such a manner to form a current mirror. Thus, assuming that the first and second driving TFTs T1 and T2 have the same channel width, a current amount flowing in the first driving TFT T1 is set to be equal to a current flowing in the second driving TFT T2.
An operation procedure of such a light-emitting cell driving circuit 30 will be described in detail with reference to a driving waveform of FIG. 4 below.
First and second gate signals SP1 and SP2 having the same width are applied to the odd-numbered electrode line GLo and the even-numbered electrode line GLe making the same horizontal line, respectively, in such a manner to overlap with each other during a predetermined period. Herein, the second gate signal SP2 is applied prior to the first gate signal SP1.
If the first and second gate signals SP1 and SP2 are supplied, then the first and second switching TFTs T3 and T4 are turned on. As the first and second switching TFTs T3 and T4 are turned on, a video signal from the data electrode line DL is applied, via the first and second switching TFTs T3 and T4, to the gate terminals of the first and second driving TFTs T1 and T2. At this time, the first and second driving TFTs T1 and T2 supplied with the video signal are turned on. Herein, the first driving TFT T1 controls a current flowing from the source terminal thereof (i.e., VDD) into the drain terminal thereof in response to the video signal applied to the gate terminal thereof to apply it to the light-emitting cell OLED, thereby allowing the light-emitting cell OLED to emit an amount of light corresponding to the video signal.
At the same time, the second driving TFT T2 applies a current id fed from the supply voltage line VDD, via the first switching TFT T3, to the data electrode line DL. Herein, since the first and second driving TFTs T1 and T2 form a current mirror circuit, the same current flows in the first and second driving TFTs T1 and T2. Meanwhile, the storage capacitor Cst stores a voltage from the supply voltage line VDD in such a manner to correspond to an amount of the current id flowing into the second driving TFT T2. Further, the storage capacitor Cst turns on the first driving TFT T1 using a voltage stored therein when the first and second gate signals SP1 and SP2 are inverted into OFF signals (e.g., ground potentials) to turn off the first and second switching TFTs T3 and T4, thereby applying a current corresponding to the video signal to the light-emitting cell OEL. On the other hand, since the second gate signal SP2 is inverted into an OFF signal earlier than SP1, that is, the second switching TFT T4 is turned off prior to the first switching TFT T3 in the prior art, it is possible to prevent a voltage charged in the storage capacitor Cst from being discharged into the exterior.
In practice, the conventional EL display device sequentially applies the first and second gate signals SP1 and SP2 to the odd-numbered and even-numbered gate electrode lines GLo and GLe, respectively, and applies video signals to the data electrode lines DL, thereby displaying a desired picture. However, such a conventional EL display device has a problem in that, since driving a single of light-emitting cell OELD requires two gate electrode lines at a single of horizontal line and four TFTs, aperture ratio is low. Moreover, such a conventional EL display device has two gate drivers to drive the odd-numbered gate electrode lines GLo and the even-numbered electrode lines GLe, leading to high manufacturing cost.